1. Field of the Invention
The invention relates generally to the field of clearance control or abradable materials. More particularly, the invention relates to a ceramic abradable material and, even more particularly, to a ceramic abradable coating that includes dysprosia and zirconia where the dysprosia serves the purpose of a stabilizer and also imparts a dramatic improvement in thermal shock resistance and abradability to the abradable coating compared to prior art abradable coatings.
2. Description of Related Art
In an aircraft or stationary gas turbine engine there is a host of air or gas seal locations. All of these sealing positions are significant with regards to safe and efficient engine operation. Efforts have continued over the last several years to improve efficiency and power output of new and existing turbines. Improvements in sealing between rotating and stationary parts in gas turbine engines can significantly reduce parasitic leakages and thereby improve performance by increasing both efficiency and power output. Abradable seals have been used in jet engines since the late 1960's and they are gaining ever more attention in industrial turbines as a relatively simple means to reduce gas path tip clearances. They offer clearance reductions at relatively low cost and minor engineering implications for new and fleet units. These materials are applied to the casing or shrouds of gas or steam turbine engines and are worn-in by the rotating blades during operation. The result is decreased clearances to levels difficult to achieve otherwise.
In the compressor section of engines the abradable seals are applied by thermal spraying metals or metal composite materials. Today gas path sealing on the turbine side is mostly achieved through brazed all-metallic honeycomb seals which, together with fins on shrouded turbine blades, form an effective labyrinth type seal. Recently, however, efforts have been undertaken to provide thermally sprayed, metallic high temperature sealing solutions for up to 950° C. in long term service and being used with unshrouded turbine blades. For even higher temperatures, metallic materials no longer provide the necessary oxidation and hot gas corrosion resistance and ceramic materials will have to be taken into consideration for seals operating at up to 1200° C. or even higher.
Conventional gas turbine engines may use porous ceramic abradable materials such as yttria (Y2O3) stabilized zirconia (ZrO2), YSZ. Previously, an abradable seal material made of zirconia stabilized with 6-8 wt % yttria (Y2O3) as used in standard thermal barrier coatings (TBCs) has been disclosed. Although such standard YSZ materials are basically suited for use as abradable materials, their performance in this application is far from optimum in terms of thermal shock behavior, sintering resistance, and their potential inability to be cut by unshrouded blades having untreated, bare alloy blade tips.
Other references have disclosed a multilayer abradable thermal barrier coating system that is softer and more resistive to sintering than prior art materials and comprises a ytterbia (Yb2O3) stabilized zirconia or YbSZ, in one of the ceramic layers whereby the ytterbia concentration in that ceramic layer is at least 50 wt %. These references describe abradability of this system when tested against silicon carbide (SiC) tipped blades and discloses advantages in abradability of the invention coatings over prior art YSZ such as 8 wt % yttria stabilized zirconia (8 YSZ) after having been exposed to a 1200° C. sintering treatment.
Ceramic abradable coatings require a special profile of characteristics to ensure their optimal performance as a high temperature abradable seal. One aspect that needs to be considered is that abradables, unlike thermal barrier coatings, have a requirement for relatively thick coatings to accommodate the design incursion depth on top of a certain minimum thickness. This challenges another important property of the ceramic abradable coating, namely the coating's resistance to spallation during thermal shock loading which generally decreases with increasing coating thickness. There remains a need in the art to provide a seal with improved thermal shock resistance.
Because the ceramics described above are abrasive to metallic materials, ceramic abradables generally require hard tipping of the blades rubbing against it so as to prevent excessive damage of the metallic blade tip. Typical commercial blade hard tipping solutions make use of ultra-hard ceramic materials such as cubic boron nitride (cBN) or silicon carbide (SiC) in an oxidation resistant metal matrix. While such abrasive tipping improves the cutting behavior of the blades, it is expensive to apply and an optimized ceramic abradable system should avoid the need for hard tipping by providing improved cuttability or abradability of the seal. Thus, there remains a need in the art to provide a ceramic seal that shows improved abradability.